Brain pathway Addiction to morphine and heroin can be blocked, according to Australian research, which could prove a major breakthrough in treating addicts and in pain relief treatments.

Researchers at the University of Adelaide worked with colleagues at the University of Colorado in the United States to pinpoint a key mechanism in the body's immune system that amplifies addiction to opioid drugs.

"Our studies have shown conclusively that we can block addiction via the immune system of the brain, without targeting the brain's wiring," says Mark Hutchinson from Adelaide's School of Medical Sciences.

"Both the central nervous system and the immune system play important roles in creating addiction, but our studies have shown we only need to block the immune response in the brain to prevent cravings for opioid drugs."

The results, published in the Journal of Neuroscience, reveal that a drug known as plus-naloxone, which is not yet in clinical use, will selectively block the immune-addiction response.

Shutting the effect down

The researchers say that opioid drugs such as morphine and heroin bind to immune receptors in the brain known as TLR4 which then act as amplifiers for addiction, ramping up the "reward" effect of drugs of abuse to a high degree.

The new drug automatically shuts this effect down, says Hutchinson.

"It really reduces the reward level down to the equivalent of food, sex, and hugs," he says.

Professor Linda Watkins, from the Center for Neuroscience at the University of Colorado, says the work fundamentally changes our understanding about opioids, reward and addiction.

"We've suspected for some years that TLR4 may be the key to blocking opioid addiction, but now we have the proof," she says.

The researchers believe the discovery could prove useful if plus-naloxone could become a co-formulated drug with morphine, to allow patients who need pain relief to take the drug without fear of addiction.

But it could have a second application.

"It can be used by itself potentially in addicted people to help their addiction," says Hutchinson.

The research team says clinical trials could be underway within 18 months.